Kaylee Frye, Firefly and Serenity

Kaylee was basically born with a tool set in her hands, growing up with a love for working on machines and dissecting exactly what makes them tick. In Whedon’s Firefly, she is hired as the ship Serenity's mechanic after she gets it flying again, even though it was declared a goner by the former mechanic. Her bubbly personality is contagious (unless you're Mal, but what’s new?), and she provides unstoppable good humor along with nearly infallible mechanical expertise.